Not much has stayed secret about the tumultuous private life of Benito Mussolini. That is apart from the true nature of his relationship with the woman who was to be Italy's last queen, Marie-José of Belgium.

Mussolini's mistress, Claretta Petacci, claimed in her diary that in 1937 the then princess and wife of the heir to the throne tried and failed to seduce the dictator at a beach resort near Rome.

But Marie-José may have been more successful than her rival suspected, if evidence that emerged on Wednesday is to be believed.

In a letter reproduced by the weekly magazine Oggi, Mussolini's son Romano quotes his mother as saying that there was a "brief period of intimate romantic relations between my father and the then princess of Piedmont".

The daughter of the Belgian king, Albert I, Marie-José was born in 1906. While still a child, it was decided that she should marry into the Italian royal family and in 1930 she wed Umberto of Savoy, the only son of King Victor Emmanuel.

By her own subsequent account, the marriage was not a happy one, and she separated from her husband after the Italian monarchy was abolished by referendum in 1946. She lived for most of the rest of her life in Switzerland where she died in 2001.

In contrast to the Savoy family, Marie-José had little time for fascism and during the Second World War made a failed attempt to broker a peace treaty with the United States.

But she did, it would seem, have time for Mussolini. It has long been known that the Italian dictator was compulsively promiscuous: by one account, he had sex with at least one woman a day at his office in Palazzo Venezia for almost 14 years until the collapse of his regime in 1943.

According to Petacci's diaries from the period, Mussolini told her that the princess had tried to seduce him at Castelporziano, a coastal area south of Rome where the king had made available to him a hunting estate in which Mussolini entertained many of his lovers.

She quoted the fascist leader as saying: "Marie-José came and said 'May I?' Then, with a small movement her dress fell and she was there virtually naked."

But she records Mussolini reassuring her that he found the princess "repulsive" and that she had made "no impression on me at all".

That is not the picture, though, that emerges from a letter written by Mussolini's youngest son in 1971 to Antonio Terzi, a former deputy editor of the newspaper Corriere della Sera.

It reads: "I can confirm in all good faith that the romantic and political relations between Marie-José and my father were often talked about at home, and I can tell you with honesty that my mother (albeit with understandable reservations) was always pretty explicit: there was a brief period of intimate romantic relations between my father and the then Princess of Piedmont that was then I believe interrupted at the instance of my father."

The report on Wednesday in Oggi said the letter was found among the journalist's papers by his son and that Romano Mussolini's widow had judged it authentic.

Though discredited by his support for Mussolini, King Victor Emmanuel clung on to his throne after the dictator's fall, only abdicating in favour of Umberto in May 1946. Umberto ruled for only 34 days, earning for himself the sobriquet "the May King".